Functional plasticity of mitochondrion-rich cells in the skin of euryhaline medaka larvae (Oryzias latipes) subjected to salinity changes

A noninvasive technique, the scanning ion-selective electrode technique (SIET) was applied to measure Na⁺ and Cl^- transport by the yolk-sac skin and individual mitochondrion-rich cells (MRCs) in intact medaka larvae (Oryzias latipes). In seawater (SW)-acclimated larvae, significant outward Na⁺ and Cl^- gradients were measured at the yolk-sac surface, indicating secretions of Na⁺ and Cl^- from the yolk-sac skin. With Na⁺ pump immunostaining and microscopic observation, two groups of MRCs were identified on the yolk-sac skin of SW-larvae. These were single MRCs (s-MRCs), which do not have an accompanying accessory cell (AC), and multicellular complex MRCs (mc-MRCs), which usually consist of an MRC and an accompanying AC. The percentage of mc-MRC was ~60% in 30 parts per thousand of SW, and it decreased with the decrease of external salinity. By serial SIET probing over the surface of the MRCs and adjacent keratinocytes (KCs), significant outward fluxes of Na⁺ and Cl^- were detected at the apical opening (membrane) of mc-MRCs, whereas only outward Cl^- flux, but not Na⁺ flux, was detected at s-MRCs. Treatment with 100 μM ouabain or bumetanide effectively blocked the Na⁺ and Cl^- secretion. Following freshwater (FW) to SW transfer, Na⁺ and Cl^- secretions by the yolk-sac skin were fully developed in 5 h and 2 h, respectively. In contrast, both Na⁺ and Cl^- secretions downregulated rapidly after SW to FW transfer. Sequential probing at individual MRCs found that Na⁺ and Cl^- secretions declined dramatically after SW to FW transfer and Na⁺/Cl^- uptake was detected at the same s-MRCs and mc-MRCs after 5 h. This study provides evidence demonstrating that ACs are required for Na⁺ excretion and MRCs possess a functional plasticity in changing from a Na⁺/Cl^--secreting cell to a Na⁺/Cl^--absorbing cell.